Add a version annotation regarding the urlopen(proxies={...}).

Doc/lib/liburllib.tex

@@ -107,6 +107,8 @@ use \class{URLopener}, or a subclass such as \class{FancyURLopener}.
 
 Proxies which require authentication for use are not currently
 supported; this is considered an implementation limitation.
+
+\versionchanged[Added the \var{proxies} support]{2.3}
 \end{funcdesc}
 
 \begin{funcdesc}{urlretrieve}{url\optional{, filename\optional{,